Depends on how you see it, you can consider multiple different options:
In my opinion: 3, Aizen, Muramasa (zampaktous filler), Jin (bounts filler), Ganryū (movie Memories of Nobody), Kusaka (movie, The Diamond Dust Rebellion), Shizuku and Homura (movie, Fade to Black), that guy from hell (movie Hell Chapter) and that guy from the princess arc filler wich i don't remember the name.
Other chars u can also see as main villains:
-the hollow version of Ichigo (often called as Ogihci, but as said by him he has no name), but still it's just an inner conflict in Ichigo
-Gin and Tosen, as they are the main alies of Aizen
-Any of the espada
-Yamamoto, in the initial episodes the soul society was Ichigo's enemy and as he's the Leader
-*Warning spoilers on this answer, if u just watch the anime may not read* Urahara, as said by Kugo Ginjo that he's no good and been using Ichigo